Breastfeeding and the amenorrhea related to breastfeeding are important determinants of fertility levels. Because of this, it is important in demographic research on fertililty to measure precisely the length of amenorrhea. However, most data on amenorrhea are from retrospective studies and are of poor quality. Models of amenorrhea have been developed to adjust available data and for use in fertility models. However, a major problem in the evaluation of the availabe models has been a lack of data to evaluate the models. The aim of this project is to examine disributions of the length of postpartum amenorrhea in three large prospective studies of breastfeeding women to examine the true distribution of amenorrhea and to evaluate the ability of the models to produce realistic distributions.